Anyone who has done a significant amount of interior or exterior painting knows preparation for the application of paint will make a huge difference in the quality and appearance of a paint job. Every crack or gap that gets filled is one more thing the eye will not be drawn to, when the job is completed. In the end, it is as much what is not seen as it is what is seen.
Note, for example, painted trim and molding inside many homes. Some have had the gap between the molding and the wall caulked. Others have not. Ones that have not been caulked have what show as dark lines the eye is drawn to. Those lines really stand out. Merely filling that gap with white caulk, even without paint (e.g. applying just enough caulk to fill the gap and wiping most off with a damp rag), makes a huge difference. It's no different on an exterior wall.
In addition to the appearance, caulked exterior walls have another advantage over walls without it - the caulking reduces areas where moisture can get behind trim and siding and damage even a good paint job.
In cold weather, water can freeze in cracks and cause them to expand more. Of course, the water can be absorbed by the wood and freeze too, causing damage. Clearly, caulking well, with good caulk is a worthwhile investment, in time and materials.
When applying caulk, one of the significant problems is, people, including pro's, use their fingers to spread caulk. Because of this, they leave smooth spots on, otherwise, rough surfaces, like cedar or other boards. I even see it on Hardiplank and other man made products. Like the shadows your eyes are drawn to, your eye is drawn to the out of place smooth spots on the rough surface.
There is one more problem regarding the application of caulk to rough surfaces - it's like dragging your finger over sandpaper. By the end of the day, every finger is raw, and your thumbs too.
To avoid the problem, I tried cotton gloves moistened in water. It worked to avoid rubbing my fingers raw, but it still resulted in smooth spots. Later, I began cutting the fingers off the gloves and carrying them with me, so I had a supply when the one I was using wore out. Again, that helped with the problem of raw fingers, but not with the problem of leaving smooth spots on rough surfaces.
Step 1: Saving Your Fingers & Matching the Wood Grain
While working a job I knew would use at least two cases of caulk, I grabbed a couple chip brushes (those cream colored, disposable brushes you pick up at places like Harbor Freight) from my van, to use in place of my fingers, for the stated reasons.
Initially, they did poorly, because the bristles were too flexible. On a whim, I cut the length of the bristles down to about one inch (1"). This left the bristles stiff enough to work the caulk into cracks and things, but still flexible enough to follow the wood pattern.
Using these brushes to apply caulk, I was able to fill gaps, but no longer had to worry about the smooth spots, on a rough surface. Of course, my fingers no longer got worked raw either.
Step 2: Using Modified Chip Brushes to Apply Caulking
1) Fill a small container (e.g., sour cream container, cottage cheese container) about 1/4 the way full of water.
2) Dampen a rag until it's almost dripping wet. (This is just to give you a place to wipe excess caulk and to clean your fingers and hands from time to time.
3) Lay on the caulk, getting as much into the hole or gap you are working as you can.
4) Using the modified chip brush, feather the caulk out over the hole or crack.
5) If the brush starts dragging or gumming up, clean it in the water by merely swirling it and taping it against the bottom, until usable again, then flick the water off and continue the job.